Variable area flow meter

ABSTRACT

A variable area flow meter for measuring the volumetric flow of a fluid flowing through the flow meter includes a body portion having a fluid inlet, a fluid outlet portion and a channel. A fixed boundary is defined between the channel and the fluid outlet portion. A fluid flow path extends between the fluid inlet and fluid outlet. A piston is arranged within the channel and longitudinally displaceable by the flow of fluid. The cross-sectional area of a portion the fluid flow path is variably determined by the structural shape of the piston and the instantaneous location of the piston relative to the fixed boundary. The longitudinal displacement of the piston in the direction of the fluid flow and consequently the cross-sectional area of the portion of the fluid flow path are dependent on the volumetric flow rate of the fluid.

RELATED APPLICATIONS

The present application claims priority from United Kingdom PatentApplication No. 0315497.8 filed Jul. 2, 2003, the disclosure of which isincorporated herein by reference in its entirety.

FIELD OF INVENTION

This invention relates to flow meters for measuring the flow rate of afluid such as a liquid or gaseous medium. The invention particularlyrelates to variable area flow meters.

BACKGROUND OF THE INVENTION

Generally, flow meters comprise a flow channel and a dynamic barrierarranged to slide within the channel. As fluid flows through the channelthe pressure of the fluid acts on the barrier so that it is displaced.The degree of displacement of the barrier is proportional to the rate ofvolumetric flow of the fluid.

The dynamic barrier is biased to return back to its original position.The biasing force may be provided by the weight due to gravity of thebarrier, magnetic means or spring means.

A variable area flow meter is designed to enable the cross-sectionalarea of the flow path to vary as the flow rate of fluid varies. Thisfeature enables variable area flow meters to measure a greater range offlow rates and to particularly measure low rates of volumetric flow. Thecross-sectional area of the flow path generally increases as the flowrate increases. This is typically achieved by using a taperedcylindrical flow channel that widens as it extends towards the outlet ofthe flow channel. These types of variable area flow meters are very wellknown and commonly referred to as a “rotameter”. Alternatively, the flowchannel may be formed with a tapered longitudinal recessed groove whichwidens in cross-sectional area as it extends from the inlet end to theoutlet end of the channel. Thus, as the flow rate increases thedisplacement of the dynamic barrier relative to the flow channelincreases and so the cross-sectional area of the flow path increases.

Patent document U.S. Pat. No. 4,466,293 (HUHTALA) describes a variablearea flow meter comprising a cylindrical flow channel (3), an inletpoint (10), an outlet point (12) and an axially movable, spring loadedindicator piston (4) which is shifted by the medium flowing through thechannel to different positions depending on the flow quantities. Aninclined outflow slot (27) is provided in the flow channel, the slotbecoming deeper as it extends from the inlet point to the outlet point.The cross-sectional area of the flow path is formed between theindicator piston and outflow slot and it varies in accordance with theposition of the indicator position.

A similar arrangement is disclosed in U.S. patent document U.S. Pat. No.4,489,614 (deFASSELLE et al) which describes a variable flow metercomprising a body portion in which a flow path extends along a core tubebetween an inlet and an outlet. A first piston is vertically mountedwithin the core tube such that it may be displaced in accordance withthe rate of flow of liquid along the flow path. A tapered recessedgroove is formed within the inner surface of the core tube and thecross-sectional area of the fluid path is defined by the groove andfirst piston.

According to the an aspect of the invention, a variable area flow meterfor measuring the volumetric flow of a fluid flowing through the flowmeter comprises: a body portion having a fluid inlet, a fluid outletportion and a channel and defining a fixed boundary between the channeland the fluid outlet portion; a fluid flow path extending between thefluid inlet and fluid outlet; a piston arranged within the channel andlongitudinally displaceable by the flow of fluid; wherein thecross-sectional area of a portion the fluid flow path is variablydetermined by the structural shape of the piston and the instantaneouslocation of the piston relative to the fixed boundary whereby thelongitudinal displacement of the piston in the direction of the fluidflow and consequently the cross-sectional area of the portion of thefluid flow path are dependent on the volumetric flow rate of the fluid.

Preferably, the cross-sectional area of the portion of the fluid flowpath increases as the displacement of the piston increases in thedirection of the flow of fluid.

Preferably, at least one longitudinally extending recess is formed in anouter surface of the piston and the said portion of the fluid path isdefined by the fixed boundary and the at least one tapered recess. Theat least one recess may extend longitudinally in a V-shape such that thewidth of the at least one recess decreases in the direction of the flowof fluid.

Furthermore, the depth of the at least one recess may vary in thedirection of the flow of fluid. Optionally, the depth of the at leastone recess increases in the direction of the flow of fluid or the depthof the at least one recess decreases in the direction of the flow offluid.

Preferably, a lower portion of the at least one recess is flat.Alternatively, a lower portion of the at least one recess is curved.

Alternatively, the piston may comprise a hollow cylinder with a closedleading end and at least one aperture is formed in the cylinder wall,wherein the said portion of the fluid path is defined by the fixedboundary, the at least one aperture and the hollow cylinder.

The at least one aperture may be V-shaped and extends longitudinallyalong the cylinder wall such that the width of the at least one aperturedecreases in the direction of the flow of the fluid. Or the at least oneaperture is V-shaped and extends longitudinally along the cylinder wallsuch that the width of the at least one aperture increases in thedirection of the flow of the fluid. Alternatively, the at least oneaperture is rectangular in shape and extends longitudinally along thecylinder wall.

Preferably, the channel further comprises a pressure relief regionarranged directly above the portion the fluid flow path and adjacent thefixed boundary of the channel.

The cross-sectional width of the piston may vary in the direction of theflow of fluid.

Preferably the flow meter further comprises cleaning means for cleaningthe variable area flow meter. The cleaning means may comprise a cleaningmembrane arranged to move longitudinally within the channel so as toclean an internal surface of the channel. The cleaning means maycomprise a cleaning membrane arranged to clean the piston as the pistonis longitudinally displaced. The variable area flow meter may optionallycomprise biasing means operative to urge the piston in a directionopposite to the displacement of the piston resulting from the fluidflow. The biasing means may include at least one of a spring, repellingpoles of a magnet and gravity.

Finally, the variable area flow meter may preferably comprise indicatormeans for indicating the volumetric flow rate of a fluid in response tothe longitudinal displacement of the piston along the channel in thedirection of the flow of fluid.

It has been found that it is significantly cheaper and easier to produceand maintain a flow meter if the piston is structurally shaped such thatthe cross-sectional area of the portion of the fluid flow path increasesas the displacement of the piston increases in the direction of the flowof the fluid. For example, it is cheaper and easier to form a shapedpiston than form recesses on an inner surface of the body.

The present invention advantageously lends itself to a modular designmuch more easily than the prior art. Flow meters having recesses formedin the body are often stored and sold as pre-assembled units, whereasflow meters with a shaped piston may be advantageously stored and soldas separate parts.

Pre-assembled flow meters of the prior art are limited in use since theyare only able to determine the flow rate of fluids with similarparameters. In contrast, the modular design of the present invention isable to measure a range of different types of fluids and a range ofdifferent types of flow rates because the piston may be swapped to onethat has the most appropriate type of shape in accordance with the fluidand flow rate to be tested. Furthermore, it is obviously cheaper andeasier to exchange a piston for another piston with a different shapethan to change the main body of the flow meter.

It is has been found that a flow meter can be much more compact in sizeif the piston is shaped rather than the body.

Finally, flow meters with at least one recess in the inner surface ofthe stationary body are very difficult to keep clean since the cleaningmechanisms are often unable to easily and efficiently access and removedirt or debris from within the recess. In contrast, it has been foundthat dirt and debris that collects on or within the shaped piston may beremoved much more easily because the piston moves axially within theflow channel relative to a cleaning mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how itmay be carried into effect, reference shall now be made by way ofexample to the accompanying drawings, in which:

FIG. 1 depicts a sectional view of a first embodiment of a flow meteraccording to the present invention.

FIG. 2 depicts a sectional view of a second embodiment of a flow meteraccording to the present invention.

FIG. 3 depicts a sectional view of a third embodiment of a flow meteraccording to the present invention.

FIG. 4 depicts a piston of a flow meter, according to the presentinvention, with a tapered recess.

FIG. 5 depicts a piston of a flow meter, according to the presentinvention, with a plurality of tapered recesses.

FIG. 6 depicts the cross-sectional area of a flow path in a flow meter,according to the present invention, when measuring a low volumetric flowrate of fluid.

FIG. 7 depicts the cross-sectional area of a flow path in a flow meter,according to the present invention, when measuring a high volumetricflow rate of fluid.

FIG. 8 depicts a cleaning mechanism for a flow meter according to thepresent invention.

FIG. 9 depicts an overview of a piston of a flow meter, according to thepresent invention, with a wide-angled V-shaped recess.

FIG. 10 depicts an overview of a piston of a flow meter, according tothe present invention, with a narrow-angled V-shaped recess.

FIG. 11 depicts a cross-sectional view of a piston of a flow meter,according to the present invention, with two shallow, flat-bottomed,tapered recesses.

FIG. 12 depicts a cross-sectional view of a piston of a flow meter,according to the present invention with two deep, flat-bottomed, taperedrecesses.

FIG. 13 depicts a cross-sectional view of a piston of a flow meter,according to the present invention, with two curved-bottomed, taperedrecesses.

FIG. 14 depicts a cross-sectional view of a piston of a flow meter,according to the present invention with four shallow, flat-bottomed,tapered recesses.

FIG. 15 depicts a sectional view of a piston of a flow meter, accordingto the present invention, with two sloping recesses.

FIG. 16 depicts a sectional view of a piston of a flow meter, accordingto the present invention, with two sloping recesses.

FIG. 17 depicts a sectional view of a hollow piston of a flow meter,according to the present invention, with a sloping recess.

FIG. 18 depicts a sectional view of a piston of a flow meter, accordingto the present invention, with a variable cross-sectional area, a curvedtip and two tapered recesses.

FIG. 19 depicts a view of the piston of a flow meter, according to thepresent invention, as depicted in FIG. 18.

FIG. 20 depicts a cross-sectional profile a piston of a flow meter,according to the present invention, wherein the piston is a hollowcylinder with two rectangular shaped apertures formed in the cylinderwall.

FIG. 21 depicts an overview of a piston of a flow meter, according tothe present invention, wherein the piston is a hollow cylinder with aV-shaped aperture formed in the cylinder wall.

FIG. 22 depicts a cross-sectional profile of a piston of a flow meter,according to the present invention, wherein the piston is a hollowcylinder with a V-shaped aperture extending from one end of the piston.

FIG. 23 depicts the cross-sectional area of a flow path in a flow meter,according to the present invention, when measuring a low volumetric flowrate of fluid using a piston as depicted in FIG. 22.

FIG. 24 depicts the cross-sectional area of a flow path in a flow meter,according to the present invention, when measuring a high volumetricflow rate of fluid using a piston as depicted in FIG. 22.

FIG. 25 depicts a cross-sectional area of a flow path in a flow meter,according to the present invention, when measuring a low volumetric flowusing a piston with a V-shaped recess that tapers in the oppositedirection to the flow of fluid.

FIG. 26 depicts a cross-sectional area of a flow path in a flow meter,according to the present invention, when measuring a high volumetricflow using a piston with a V-shaped recess that tapers in the oppositedirection to the flow of fluid.

DETAILED DESCRIPTION OF THE INVENTION

The flow meter illustrated in the Figures consists of a body (1) throughwhich fluid may flow. The body may be formed from a plastic materialand/or a metallic material such as steel. The plastic material may beopaque or transparent.

The body comprises a fluid inlet (2), a fluid outlet portion (3) and achannel (4). The fluid inlet (2) and fluid outlet portion (3) areessentially connected by the channel (4).

The fluid inlet (2) and fluid outlet portion (3) may be arranged inparallel or perpendicular to the longitudinal axis of the channel.Furthermore, the fluid inlet (2) and fluid outlet portion (3) may bearranged at opposite ends of the body or part way along the body. FIGS.1 and 3 depict a flow meter where the fluid inlet is mounted in asidewall of the body in parallel to the longitudinal axis to the channeland the fluid outlet portion is arranged along an adjacent wall at theother end of the body perpendicular to the longitudinal axis of thechannel. FIG. 2 depicts a flow meter where the fluid inlet and fluidoutlet portion are arranged parallel to the longitudinal axis of thechannel in sidewalls at opposite ends of the body.

The fluid outlet portion (3) may include at least one outlet guidechannel (3 a) to guide fluid out of the body (1).

The channel (4) is preferably, though not essentially cylindrical inshape.

A fixed boundary (10 a) occurs between the channel (4) and fluid outletportion (3). The boundary is a notional line defining the junction ofthe channel and the outlet portion (3). As discussed below, the fluidoutlet portion (3) may be the outlet itself of may include a pressurerelief region (11). As such, the boundary may be the junction of thechannel with the outlet itself or the pressure relief zone.

The flow meter further comprises a piston (5) that is arranged to movelongitudinally along the channel (4) relative to the fixed boundary (10a). The piston (5) is displaced in the direction of the flow of fluid ifa sufficient force is exerted by the fluid as it flows through the flowmeter. The displacement of the piston in the direction of the flow offluid relates directly to the volumetric flow rate of the fluid.

The structural shape of the piston (5) varies along its longitudinallength. This may be achieved by providing at least one recess oraperture.

At least one recess (6) may be formed on the outer surface of the piston(5). FIGS. 1, 2, 3 and 4 depict a piston with only one recess, whereasFIGS. 5, 11 to 16 depict pistons with a plurality of recesses. Therecesses are V-shaped and taper in the direction of the flow of fluid.The width of the V-shape recess preferably decreases in the direction ofthe flow of fluid such that the widest point of the recess is formed atthe fluid inlet end of the piston and the narrowest point is formed atthe fluid outlet end of the piston. The sidewalls of the V-shaped recessmay taper linearly or in a curved manner. The lower portion of therecess may be flat or curved. The recess may alternatively be referredto as a “slot” or “groove” formed in the outer surface of the piston.The piston shaped to include a least one recess may be solid or a hollowcylinder with a closed trailing end.

Alternatively, the piston (5) may be a hollow cylinder with a closedleading end with at least one aperture (13) formed in the wall of thecylinder. The apertures may be V-shaped, rectangular or lozenge-shaped.If the apertures are V-shaped then they may arranged such that the widthof the aperture either increases or decreases in the direction of theflow of fluid. FIG. 20 depicts a cross-sectional profile of a hollowpiston with two rectangular apertures. FIG. 21 depicts an overview of ahollow piston with a single V-shaped aperture arranged to taper in thedirection of the flow of fluid. FIG. 22 depicts an overview of a hollowpiston with a single V-shaped aperture that extends from one end of thepiston and tapers in the direction of the flow of fluid.

As the fluid flows through the flow meter it follows a fluid flow path.The fluid flow path extends between the fluid inlet and fluid outletportion. A portion of the fluid flow path may be defined by the boundaryof the channel and at least one recess. The cross-sectional area of thisportion of the fluid flow path is determined by the shape of the recessand the position of the piston relative to the boundary. Thecross-sectional area of this portion of the fluid path varies as thepiston is displaced within the channel relative to the boundary. Therecesses on the piston are arranged to taper in the direction of theflow of fluid such that the cross-sectional area of the portion of thefluid flow path increases as the piston is displaced by the forceexerted by the fluid. Alternatively, a portion of the fluid flow pathmay be defined by the boundary, at least one aperture and the hollowcylinder. The cross-sectional area of this portion of the fluid flowpath is determined by the shape of the aperture, the hollow cylinder andthe position of the piston relative to the boundary. Again, thecross-sectional area of the portion varies as the piston is displacedwithin the channel relative to the boundary. The at least one apertureformed in the cylinder wall of the piston are arranged such that thecross-sectional area of the portion preferably increases as the pistonis displaced by the force exerted by the fluid.

The body may include a bush (10) against which a portion of the pistonslides as it is displaced within the channel. The piston is arrangedsuch that it always extends longitudinally beyond the bush. The edge ofthe bush proximate the outlet end of the channel may act as the boundaryof the channel.

The channel may further include a pressure relief region (11) defined bythe boundary, the internal surface of the channel and the at least onerecess. Alternatively, the pressure relief region may be defined by theboundary, the internal surface of the channel and the at least oneaperture. The pressure relief region (11) may be formed in the fluidoutlet portion (3).

FIGS. 6 and 7 show how the cross-sectional area of the fluid flow path,defined by the at least one V-shaped recess and boundary, varies as thepiston is displaced relative to the boundary. FIG. 6 depicts thedisplacement of the piston when a fluid with a relatively low volumetricflow rate flows through the flow meter. Although the piston has moved tothe left due to the fluid force, the width W1 across the V-shaped recessremains reasonably small and so the increase in the cross-sectional areaof the fluid path is small. However, if the volumetric rate of flow isrelatively high then the piston is pushed much further to the left (seeFIG. 7) and so the width W2 of the recess significantly increases whichleads to a significant increase in the cross-sectional area of the fluidpath. Obviously, if the cross-sectional area increases then the volumeof the fluid flow path defined by the recess, boundary and pressurerelief region increases as the piston is displaced due to an increase involumetric rate of flow.

FIGS. 23 to 26 show how the cross-sectional area of the fluid path,defined by the at least one aperture, hollow cylinder and boundary,varies as the piston is displaced relative to the boundary. FIG. 23depicts the displacement of a piston, with a V-shaped aperture thatdecreases in width in the direction of the flow of fluid, when the fluidhas a relatively low volumetric rate of flow and shows how thecross-sectional area Al of the fluid flow path is relatively small.However, if the flow rate increases, then the piston is pushed furtherto the left and so the cross-sectional area A2 of the portion of thefluid flow path increases. Likewise, FIGS. 25 and 26 show how thecross-sectional area of the fluid flow path increases when the rate offlow increases using a piston with a V-shaped aperture that increases inwidth in the direction of the flow of fluid.

Flow meters often become blocked due to impurities, dirt and debriscarried by the fluid. For example, impurities in a liquid medium such ashumus substances in water may form sediments on the internal surfaces ofthe channel. Also, solid particles may accumulate in the recesses. Asdirt collects the fluid flow path becomes restricted and the flow meterbecomes increasingly inaccurate. Therefore, the flow meter may includecleaning means to help remove impurities, dirt and debris. FIGS. 1, 2and 3 depict a cleaning membrane (7) that is arranged within the channelof the flow meter between the fluid inlet and piston. FIG. (8) shows howthe cleaning membrane includes holes (7 a) so that the fluid can passthrough freely and its flow it not hampered. The cleaning membrane isarranged such that it may be moved independently along the channel usinga handle (7 b). As the cleaning membrane slides along the channel theouter ring (7 c) removes sediments and dirt that has collected on theinternal surface of the channel. The removed debris is then carried bythe fluid flowing through the flow meter and out through the fluidoutlet. FIGS. 1, 2, and 3 also depict a cleaning membrane (7′) that isarranged within the channel between the piston and the fluid outlet.This particular cleaning membrane is permanently mounted with respect tothe piston such that it can clean the outer surface of the piston, thatat least one recess formed on the outer surface of the piston or mayclean the inside of the piston if it is a hollow cylinder with at leastone recess as the piston is displaced along the channel. Again, theunwanted debris is carried by the fluid towards the fluid outlet.

The flow meter may include biasing means (9) to provide a biasing forceon the piston. The biasing force pushes the piston back to its originalstarting position if fluid stops flowing through the flow meter. Thebiasing means may include a coiled spring (91) as depicted in FIGS. 1and 2, and/or may include the repelling poles of a magnet (92, 93) asdepicted in FIG. 3. Furthermore, if the flow meter is arrangedvertically then the weight of the piston due to the force of gravityacts as a biasing force.

Indicator means may work in conjunction with the piston in order totranslate the displacement of the piston into a readable value of thevolumetric flow of the fluid. The flow meter may include a linearmeasuring scale arranged relative to the piston such that a user may beable to measure the displacement of the piston and determine thesubsequent volumetric flow rate of the fluid.

FIGS. 1 to 3, 6 & 7 show how a linear measuring scale (12) may bealigned on the body of the flow meter alongside the piston to indicatethe rate of flow. In this particular example, a portion of the body istransparent so that the displacement of the piston within the channelmay be detected and measured by the linear scale. As discussed above,the transparent portion of the body may be formed from transparentplastic. The flow meter may optionally or alternatively include ameasuring dial that works in conjunction with the piston to indicate thetotal displacement of the indicator piston/volumetric flow rate of thefluid.

The operation of a variable area flow meter should now be readilyapparent. Essentially, fluid flowing through the flow meter creates afluid pressure that exerts a force on the piston. The piston isdisplaced in the direction of the flow of fluid until thecross-sectional area of a portion of the fluid flow path andconsequently the volume of the fluid flow path, increases until it issufficiently large enough to release the fluid pressure so that the flowmeter reaches equilibrium. The volume of the portion of the fluid flowpath equates to the volumetric flow of fluid when equilibrium occurs.Thus, the final displaced position of the piston indicates thevolumetric flow rate of the fluid.

The number of recesses, angle of the V-shape, depth of the groove andcross-sectional profile of a recess has an effect on the pressuregenerated and released within the flow meter, the range of volumetricflow rates the flow meter can determine and the range of fluids a flowmeter can measure. Thus, flow meters must be designed with the mostappropriate type of recesses for measuring certain types of fluids inparticular circumstances.

Also, the number of apertures, length of an aperture, angle of theV-shape or width of rectangle has an effect on the pressure generatedwithin the flow meter, range of volumetric flow rates the flow meter candetermine and the range of fluids a flow meter can measure. Accordingly,flow meters must be designed with the most appropriate type of aperturefor measuring certain types of fluid in particular circumstances.

As discussed above, prior art variable area flow meters, having recessesor apertures formed in the body, are often stored and sold aspre-assembled units whereas the flow meters depicted in the Figures havea modular design such that they may be stored and sold as separateparts. The prior art models are limited to the testing fluids withsimilar parameters and are limited to a small range of flow rates. Thisis because the body cannot be swapped to another having more appropriaterecesses in accordance with the fluid and flow rate to be tested. Thepresent invention provides a flow meter that is able to determine thevolumetric flow rate for a greater range of fluids under different typesof conditions because the piston may be easily exchanged for a pistonwith more suitable recesses or apertures so that the flow meter canprovide a more accurate reading.

The angle of the V-shaped recess or apertures may be varied so thatdifferent flow rates of different fluids may be more accuratelymeasured. FIGS. 9 and 10 depict pistons with different sized V-shapedrecesses. FIG. 9 depicts a piston with a wide angled V-shaped recesswhilst FIG. 10 depicts a piston with a narrowed angled V-shaped recess.The angle of the V-shaped recess or aperture may range from 1 to 89°.However, it is preferable for the V-shaped recess or apertures to rangefrom 5 to 70°. The size of angle is dependent on the length of therecess or aperture and the length and/or cross-sectional width of thepiston.

FIGS. 11 and 12 depict the cross-sectional profile of a piston with tworecesses. The recesses of the piston in FIG. 11 have a shallow depthwhilst the recesses in the piston shown in FIG. 12 are much deeper.

Both sets of recesses in FIGS. 11 and 12 have a lower portion that isflat. Whilst the recesses of the piston depicted in FIG. 13 have acurved lower portion.

FIG. 14 depicts a cross-sectional profile of a piston that has four,shallow recesses with flat lower portions.

The recesses may also taper by sloping longitudinally in the directionof the flow of fluid. FIG. 15 depicts a piston in which the recessesincline at an angle in the opposite direction to the flow of fluid,whilst FIG. 16 depicts a piston in which the recesses incline at anangle in the direction of the flow of fluid. The angle of inclinationmay range from 0 to 90°. However, it is preferable for the angle ofinclination to range from 5 to 70°. Again, the angle of inclinationdepends on the length of the recess and the length and/orcross-sectional width of the piston.

FIG. 17 depicts a hollow piston with a closed trailing end with only onerecess. Obviously, a hollow piston is much lighter weight than a solidpiston. Biasing means such as a spring and/or magnet may be placedwithin the hole of the piston.

The clearance gap between the bush and piston may need to be varied inaccordance with the type of fluid being measured. This may be achievedby using pistons of different cross-sectional widths for differentfluids and/or by tapering the cross-sectional width of a piston. FIGS.18 and 19 depicts a piston where the width of the piston varies alongits length and the tip of the piston at the fluid inlet end curvesaerodynamically.

1. A variable area flow meter for measuring the volumetric flow of a fluid flowing through the flow meter comprising: a body portion having a fluid inlet a fluid outlet portion and a channel and defining a fixed boundary between the channel and the fluid outlet portion; a fluid flow path extending between the fluid inlet and fluid outlet; a piston arranged within the channel and longitudinally displaceable by the flow of fluid; wherein the cross-sectional area of a portion the fluid flow path is variably determined by the structural shape of the piston and the instantaneous location of the piston relative to the fixed boundary whereby the longitudinal displacement of the piston in the direction of the fluid flow and consequently the cross-sectional area of the portion of the fluid flow path are dependent on the volumetric flow rate of the fluid.
 2. The variable area flow meter according to claim 1 wherein the cross-sectional area of the portion of the fluid flow path increases as the displacement of the piston increases in the direction of the flow of fluid.
 3. The variable area flow meter according to claim 1 wherein at least one longitudinally extending recess is formed in an outer surface of the piston and the said portion of the fluid path is defined by the fixed boundary and the at least one tapered recess.
 4. The variable area flow meter according to claim 3 wherein the at least one recess extends longitudinally in a V-shape such that the width of the at least one recess decreases in the direction of the flow of fluid.
 5. The variable area flow meter according to claim 3 wherein the depth of the at least one recess varies in the direction of the flow of fluid.
 6. The variable area flow meter according to claim 5 wherein the depth of the at least one recess increases in the direction of the flow of fluid.
 7. The variable area flow meter according to claim 5 wherein the depth of the at least one recess decreases in the direction of the flow of fluid.
 8. The variable area flow meter according to claim 3 wherein a lower portion of the at least one recess is flat.
 9. The variable area flow meter according to claim 3 wherein a lower portion of the at least one recess is curved.
 10. The variable area flow meter according to claim 1 wherein the piston is a hollow cylinder with a closed leading end and at least one aperture is formed in the cylinder wall, wherein the said portion of the of the fluid path is defined by the fixed boundary, the at least one aperture and the hollow cylinder.
 11. The variable area flow meter according to claim 10 wherein the at least one aperture is V-shaped and extends longitudinally along the cylinder wall such that the width of the at least one aperture decreases in the direction of the flow of the fluid.
 12. The variable area flow meter according to claim 10 wherein the at least one aperture is V-shaped and extends longitudinally along the cylinder wall such that the width of the at least one aperture increases in the direction of the flow of the fluid.
 13. The variable area flow meter according to claim 10 wherein the at least one aperture is rectangular in shape and extends longitudinally along the cylinder wall.
 14. The variable area flow meter according to claim 1 wherein the channel further comprises a pressure relief region arranged directly above the portion the fluid flow path and adjacent the fixed boundary of the channel.
 15. The variable area flow meter according to claim 1 wherein the cross-sectional width of the piston varies in the direction of the flow of fluid.
 16. The variable area flow meter according to claim 1 further comprising cleaning means for cleaning the variable area flow meter.
 17. The variable area flow meter according to claim 16 wherein the cleaning means comprise a cleaning membrane arranged to move longitudinally within the channel so as to clean an internal surface of the channel.
 18. The variable area flow meter according to claim 16 wherein the cleaning means comprise a cleaning membrane arranged to clean the piston as the piston is longitudinally displaced.
 19. The variable area flow meter according to claim 1 further comprising biasing means operative to urge the piston in a direction opposite to the displacement of the piston resulting from the fluid flow.
 20. The variable area flow meter according to claim 19 wherein the biasing means comprise at least one member selected from the group comprising a spring, repelling poles of a magnet and gravity.
 21. The variable area flow meter according to claim 1 further comprising indicator means for indicating the volumetric flow rate of a fluid in response to the longitudinal displacement of the piston along the channel in the direction of the flow of fluid. 